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James Webb is fully deployed

nasa.gov

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Re: James Webb is fully deployed

#481

Post by moffkalast »

willis936 wrote: post #469
moffkalast wrote: post #464

Earlier quoted context omitted.

Given that this does not really give the orbit any real significance in terms of being shaded by the Earth from the Sun, is there another reason it's there? I guess it gives you a very stable position for observations, but why not just put it in solar orbit then? Then again this is kind of a solar orbit that happens to stay close to the Earth at all times so that's a plus for comms.

That's exactly what L2 is. Otherwise the orbits of JWST and Earth would have different periods. It's handy to be able to have constant high speed communications and not need large antennas with blackouts. It also is handy to keep the closest IR sources (Earth and moon) in the same direction as the sun at all times so there is never a point where you have a significant IR source above the sun shield.

Ah yes I hadn't considered the IR sources aspect, that makes a lot of sense.

Re: James Webb is fully deployed

#482

Post by JohnBooty »

joshvm wrote: post #451
bmitc wrote: post #411

Earlier quoted context omitted.

It seems to me that that isn't quite right. "Dark" just means no visible light, but an infrared or thermal camera could see stuff. The article above mentions this possibility and the problems, which are harnessing and operating at cryogenic temperatures.

It's probably not practical for JWST, but starlight is actually fairly bright. If you travel to somewhere with essentially zero light pollution, like a desert, if it's a clear sky you can see with starlight. At the South Pole on a clear night we would walk out to the dark sector (where the telescopes are) with our headlamps off because it just wasn't necessary. The only illumination sources outside were red safety li…

    If you travel to somewhere with essentially zero light pollution, like a desert, if it's a clear sky you can see with starlight
This reminds me... I need to experience this firsthand at some point. Thank you!

Question: how much (if any) of that light is due to atmospheric scattering? ie, the sun's light hitting the daytime side of the earth, being scattered in our atmosphere, and faintly illuminating the night side of our planet? Would the same be true in the zero-atmosphere environment of space?

Here are some possible answers: https://space.stackexchange.com/questions/25901/how-bright-i...

Re: James Webb is fully deployed

#483

Post by awb »

older wrote: post #468
awb wrote: post #379

Earlier quoted context omitted.

The cost: 23 years and $10B The benefit: “JWST’s instruments are designed to make discoveries across the spectrum of astronomy — ranging from the worlds and mini-worlds in our own solar system to alien planets circling distant stars, from the supermassive black hole at the center of our own Milky Way galaxy to the edge of the observable universe.” ( https://www.geekwire.com/2021/high-cost-high-risk-high-hopes... ) Ex…

So it is cheaper than most of recent Olympics. More benefit as well.

Right, but I think the risk factor as well makes it emotional. There was a non-0 chance that it could be unrecoverable and we could get 0 use of it if one of the points of failure went wrong.

Re: James Webb is fully deployed

#484

Post by JohnBooty »

yreg wrote: post #467
JohnBooty wrote: post #349

Earlier quoted context omitted.

I just want to make sure I understand. Your position is that they should have added significant complexity to a maneuver that was already the most complex of its kind. A maneuver with $10 billion and two decades of work on the line. In order to make the first 30 days of a 10+ year mission marginally more entertaining. You propose this while fully aware that the "entertainment" in question consists of things unfolding…

I doubt adding a tiny camera is adding a significant complexity.

I see you haven't read NASA's aforelinked blog post where they explicitly detail the sorts of complexity it would add.

Since you've indicated an unwillingness to read the blog post I'll summarize briefly:

A camera on the "light" side of JWST would need its own heat shielding and wouldn't show us any of the instruments.

A camera on the "dark" side of JWST would need to function at temperatures close to absolute zero. It would somehow need to do this without disturbing the ultrasensitive instruments on the JWST.

This would have been needed to be accomplished with, essentially, 10-30 year old technology as the designs for spacecraft like this need to be locked in far ahead of time due to the incredible complexity of deep space launches.

In both cases the cameras would need power and data cable routing, and given the various harsh constraints involved (complexity, budget, liftoff weight, size) would have involved compromises in some other area.

Re: James Webb is fully deployed

#485

Post by JohnBooty »

SCAQTony wrote: post #385
dkural wrote: post #371

Earlier quoted context omitted.

Quoted post unavailable.

What if the public funded it and tolerated the cost overruns, and the numerous delays because they wanted the "circus aspect" photography? Looking at you Juno and thanks for the swirling ocean photos on Jupiter. I bet the majority of the public did! YMMV

But the JWST will be returning some spectacular images: it's a telescope.

That's why I don't understand this debate. You'll be getting lots of spectacular images from the earliest moments of the universe. Is that clear? We are getting a time machine here. We're just not getting selfies.

    What if the public funded it and tolerated 
    the cost overruns
OK. Let's assume unlimited budget. The public (or, at least, Congress) has already tolerated plenty of them on this project. So that's a very reasonable "what if."

You'd still face the hard payload constraints of the launch vehicle itself. Every kilogram and centimeter devoted to this camera system (and the associated power and shielding requirements) would represent kilograms and centimeters that would need to be chopped elsewhere. You'd need to compromise or eliminate other aspects of the spacecraft, and/or tolerate the increased complexity and risk resulting from the inclusion of a zero-science-value selfie camera.

I would expect HN readers, of all folks, to be really familiar with the concept of the harsh realities of engineering solutions for resource-constrained environments.

Anyway, the armchair spacecraft designers among us clamoring for a selfie camera might as well go full armchair spacecraft designer. Tell us which of the JWST's instruments you would have compromised or scrapped in order to accommodate the selfie camera.

Re: James Webb is fully deployed

#486

Post by JohnBooty »

dev_tty01 wrote: post #394
JohnBooty wrote: post #349

Earlier quoted context omitted.

I just want to make sure I understand. Your position is that they should have added significant complexity to a maneuver that was already the most complex of its kind. A maneuver with $10 billion and two decades of work on the line. In order to make the first 30 days of a 10+ year mission marginally more entertaining. You propose this while fully aware that the "entertainment" in question consists of things unfolding…

NASA has often added video or image capability with low scientific value to increase public engagement, despite the associated risk and cost. They made a call not to do it this time. My only point is that it would have been a nice PR opportunity if they could have found a way to make it work within their mission parameters. I apologize if making such a simple and obvious point has offended you in some way.

As the aforelinked blog post explains, the extreme environment of the JWST makes this more challenging than it would be for missions like the Mars rovers.

The dark side of the JWST (where all the instruments are) will be operating very close to absolute zero and in near-absolute darkness which, of course, is the entire point of the mission.

A selfie camera there would need to function in that environment, while also somehow not polluting the instruments on that side of the camera with heat.

It's surely do-able, but it's a more complex engineering task (and would therefore require more tradeoffs) than operating a camera in the relatively balmy environment of Mars, where heat pollution is of no concern because of the different nature of the mission.

Keep in mind that doing this on the JWST also need to have been accomplished with decades-old technology, given the extremely long design and engineering lifecycle of a complex space mission.

Re: James Webb is fully deployed

#487

Post by mturmon »

ddalex wrote: post #392
mturmon wrote: post #358

Earlier quoted context omitted.

JW does indeed circle that Lagrange point, but it is definitely not in Earth’s shadow. The JW orbit semi major axis (about the L2 point) is order of 500,000 km. The radius of Earth is about 6500 km. Thus, the shadow of the Earth is extremely small compared with the excursions of JW.

The earth's shadow never reaches L2 anyhow - it's only penumbra at that distance since the angular size of the earth is smaller then the angular size of the Sun

Good point. JW is at about 1.01 AU, and the solar radius is a bit over 100x the Earth radius, so there’s not really even a “shadow”.

Re: James Webb is fully deployed

#488

Post by colechristensen »

atleta wrote: post #260
colechristensen wrote: post #229

Earlier quoted context omitted.

No L2 is on the further side of the moon. Maybe when we have active bases on the moon or Mars would a rendezvous with L2 be feasible but not before. Hubble is in low earth orbit and far more accessible.

Not exactly. Actually it's on the opposite side of the Earth relative to the Sun. So the 3 are always on one line but it has nothing to do with the position of the Moon. (Maybe you thought it was at the L2 of Earth-Moon system?)

Oof, I knew that. JWST is at the earth-sun L2, that fact just didn’t enter my mind trying to figure out how far away it was.

Re: James Webb is fully deployed

#489

Post by rkuykendall-com »

mywacaday wrote: post #445
devenvdev wrote: post #424

Earlier quoted context omitted.

I don't understand why is it considered extreme cost. IIRC it's around $1/us citizen/year. For example the budget for keeping us safe from ourselves is much higher.

That's a great point, Makes me wonder what long term projects my own government have. Does anyone know of other inspiring long term projects in progress? The ITER fusion project is one that comes to mind.

You could probably find any by searching your country's name here: https://en.wikipedia.org/wiki/List_of_megaprojects

It seems to cover all sorts of categories, and the James Webb Space Telescope is there (though listed as still under construction).

Re: James Webb is fully deployed

#490

Post by JohnBooty »

tomr_stargazer wrote: post #373
willis936 wrote: post #350

Earlier quoted context omitted.

Actually JWST orbits around L2 at appreciable altitude. The unique aspect of the orbit is that it is never in the shadow of the Earth or moon. It guarantees consistent solar power and thermal load. https://jwst-docs.stsci.edu/jwst-observatory-characteristics...

Wow. I’m an astronomer and somehow I never learned this! Thank you for the link! Of course a solar powered satellite can’t be parked in the shade. Can’t believe I never thought about that.

My first thought is, "Why didn't they eschew solar power entirely and 'simply' park it in the shade?"

Surely they considered that, and the wins would have been massive -- no sunshields needed. So I'm sure the downsides must have been massive as well. Not enough plutonium fuel, or perhaps it just wouldn't provide enough power over the life of the mission. And of course the radioactive fuel would generate its own heat as well.

Now I need to find out more about that decision....

edit: Another commenter mentioned, "The earth's shadow never reaches L2 anyhow - it's only penumbra at that distance since the angular size of the earth is smaller then the angular size of the Sun." If that's correct, then there was never truly an option of "parking it in the shade" anyway.

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